Deformation after posing with armature
Your hips are deforming because their vertices are weighted relatively to two bones: Hips and Spine. When you move the Hips bone the vertices receive two instructions: one (Hips) is telling to move, one (Spine) is telling to stay still: the result is the combination of theese two istances. If you remove from vertices the Spine asignement everything will be as desired. While you're editing your rig I suggest you to have Hips and Spine bones going from down to top (it's a good practice, when you will start animating you'll understand why).